The present invention relates generally to a method and apparatus for detecting and measuring elastic anisotropy in samples of the earth's formations.
It has generally been acknowledged that single crystal materials can exhibit elastic anisotropy which is related to their crystallographic symmetry. Since rocks are generally an aggregate assembly of minerals, which was assumed to be random, such rocks have generally been assumed to be isotropic. If, however, the mineral orientation is not random, one would then expect crystal-like anisotropy to be manifested in the rock properties. Rocks with pronounced visual fabrics such as shale, schists, and other metamorphic rocks as well as igneous rocks such as dunite have been clearly documented as possessing elastic anisotropy which is strongly related to their visual fabric and mineral composition. There exists other more subtle factors which can also give rise to anisotropic behavior such as the presence and orientation of cracks in the rock.
Generally, anisotropy has been a complication to be ignored by explorationists except in dealing with igneous and metamorphic rocks with a pronounced mineral alignment. However, it has become evident that sedimentary rocks as well as rocks containing fractures can have pronounced and measurable elastic anisotropy. In fact, recent seismic investigations have indicated that the greater portion of the earth's crustal surface may be more anisotropic than originally believed. Consequently, previous laboratory measurements of acoustic velocities of samples from sedimentary basins, which can depend upon the presence or absence of anisotropy, are generally believed to have been in error since anisotropy, as a matter of principle, has not been taken into account. Laboratory studies concerning formation anisotropy have generally focused upon velocity analysis and have paid little attention on the amplitude dependence of anisotropy. More recently Sprunt, et al., in U.S. Pat. Nos. 4,631,963 and 4,631,964 described a method and apparatus for measuring shear wave velocity anisotropy in formation samples.
The present invention provides a novel method for accurately determining elastic anisotropy both in terms of magnitude and symmetry in formation samples which greatly simplifies and expedites the detection and measurement of such elastic anisotropy.